The U.S. Environmental Protection Agency has identified the metal finishing industry as one of the one most significant contributors to environmental pollution in the United States and, in all likelihood, throughout the rest of the world. This is because the materials currently most used in metal finishing include chromium, cadmium, zinc, lead, copper, nickel, chromates, and many toxic or polluting volatile organics.
The aircraft industry, being one of the largest of the industrial finishers, provides an example of the environmental impact of these processes. In a 1990 report, Tinker Air Force Base (Oklahoma) reported producing and treating 1.4 million gallons of industrial waste water per day mostly from metal finishing. The principal contaminants were chromium, nickel, copper, cadmium, lead, zinc, tartrates, EDTA, phosphate, and ammonia. It is estimated that the cost of disposing of these wastes is approximately $220 per ton, which equates to several thousand dollars per day for this one site.
The chromate ion, which is an excellent corrosion inhibitor, has been one of the most widely used for almost a hundred years. It is generally used as a pigment in corrosion inhibitive paints, wash primers, sealants and caulks. It is also used in chromate conversion coatings, etching solutions, and in sealing anodized and phosphate coatings.
For the past ten years, however, chromate has been recognized as toxic and carcinogenic, and because of its health risks, has become highly regulated. With pressure for elimination being exerted by government regulations, continued use of chromate will incur ever increasing economic penalties. Hence, there is urgent need for non-toxic substitutes, both from economic and environmental standpoints.
Currently, the most widely used inhibitors for passivating aerospace aluminum alloys and other light metals are the alkaline earth and zinc salts of hexavalent chromium. They vary mostly in their degree of water solubility (in the order Mg&gt;Ca&gt;Sr.gtoreq.Zn) and to a much smaller extent in their pH (Sr.gtoreq.Ca&gt;Mg). The chromate anion is the active species, reliably performing four necessary functions to be more fully described below. In addition, all of the above-described hexavalent chromium salts enhance adhesion in many paint and sealing systems.
Chromate performs four functions, thereby making it a desirable inhibitor. These functions include:
1. Rapid exit from a carrier matrix, such as paint or sealant. PA1 2. Adsorption of the chromate anion on the bare metal or metal oxide. This alters space charge distributions at the interface, lowering the isoelectric point of the protective anodic metal oxide layer naturally forming on active metals. This repels chloride attack, and/or shifts the corrosion potential of that metal and/or its pitting potential in the noble direction. PA1 3. Instead of oxygen reduction at cathode sites, reduction of chromium (VI) anion occurs to form an acid-insoluble ("persisting"), chromium (III) oxide layer at cathode sites. This fills oxide voids over cathode sites and blocks further corrosion reaction. PA1 4. Buffering the pH or neutralizing increasing acidity at the metal/electrolyte interface which comes from metal oxidization in the absence of air. Increasing acidity accelerates corrosion exponentially.
In addition to these desirable inhibitive functions, chromate salts have the advantages of: (1) promoting adhesion at the metal/resin interface under a coating or sealant compound; (2) working well on a wide variety of metal and alloy substrates because they passivate both anodically and cathodically; (3) being relatively neutral in pH; and (4) being strong oxidizers only in acid conditions, and thus not destroying or strongly reacting with the resin matrix in which they are placed.
The prior art discloses a number of non-chromate species which have some inhibitive capabilities. For example, U.S. Pat. No. 5,126,074 discloses "hydrogen phosphate" anions as exhibiting corrosion inhibitive activity on aluminum. The patent further discloses the use in coatings of alkaline earth monohydrogen phosphates, together with a carbonate of the same alkaline earth metal and an additive of alkaline fluorosilicate or fluoroborate or alkali or alkaline fluoride. This combination is said to prevent filiform corrosion on aerospace aluminum alloys.
Other references cite the dihydrogen phosphate anion as adsorbing on alumina and lowering its isoelectric point (IEP) from pH=9 to pH=5. Lowering the IEP of aluminum oxide on aluminum metal has been shown to increase its resistance to pitting. Since this species also exhibits buffering capability, it performs functions 2 and 4.
U.S. Pat. No. 2,624,708 discloses carcinogenic mercaptobenzothiazole (MBT) as an inhibitor for aluminum and steel. Sulphur and mercapto groups which are "soft bases" are known to have a high affinity for noble and other bare metal ("soft acid") surfaces. They are effective inhibitor structures under acidic conditions where no oxide is present. By itself, this species performs only function 2.
U.S. Pat. Nos. 4,457,790 and 5,125,989 disclose the use of Mannich adducts of vegetable tannin or polyalkenyl phenols to "conversion coat" aluminum. A titanium ion or compound such as fluorotitanic acid, among others, is claimed as a co-reactant. U.S. Pat. No. 5,129,967 discloses minute catalytic amounts of dihydrohexafluorotitanic acid and hydrofluoric acid used with much larger amounts of dihydrohexafluorozirconic acid and polyacrylic acid. These patents refer to usages on aluminum and/or aluminum alloys.
U.S. Pat. No. 5,314,532 discloses zinc, cobalt, nickel and lead cyanamide pigments as exhibiting corrosion inhibitive effects on silver and thin mirror coatings. Bare, oxide-free copper would be expected to show adsorption characteristics somewhat analogous to silver, especially in an acidic, crevice environment. As with silver groups, the availability of electrons on the cyano group act as a "soft base" on bare metal "soft acid" surfaces, performing function 2.
The present invention provides a corrosion-inhibiting coating composition which performs many, if not all of the same functions as a chromate-containing composition, but without the need for the harmful chromate species. The problem solved by this invention is the elimination of toxic hexavalent chromium salts which are known to be human carcinogens, as corrosion inhibitors from treatment solutions, coatings, and sealants used on aluminum and other metal alloys. The present invention provides for the synergistic combinations of non-chromate inhibitors for aerospace aluminum alloys and other metal surfaces which can be incorporated into both curable and non-curing sealants and into curable primer and unicoat systems. This synergistic combination of inhibitors can also be incorporated into water-containing or water-absorbing fluids that might cause corrosion when used in the proximity of metal such as de-icing liquids and coolants. These and other advantages of the present invention will be readily apparent from the description, discussion and examples which follow.